Various methods for growing high quality gallium nitride (GaN) crystals are being developed by laboratories across the world. However, other nitride crystals, such as AlN and InN, would be very useful as substrates for applications such as UV-LEDs, UV-photodetectors, and field-effect transistors but are not currently available. The bandgap of AlN (6.2 eV) is much larger than that of GaN (3.4 eV), making it the substrate of choice for deep-UV light emitters and detectors. In addition, the thermal conductivity is significantly higher than that of GaN, potentially making it superior as a substrate for certain electronic devices, such as field-effect transistors (also known as high electron mobility transistors). InN has the highest electron velocity of any known nitride, and may be useful for high-frequency devices.
There is substantial prior art in the growth of crystalline nitrides other than GaN, but none of it employs supercritical solvents at pressures above about 5 kbar. None of the existing approaches are yet commercially viable from the standpoints of simultaneous requirements for: (i) producing large (≧2″ in diameter), single-crystal boules suitable for slicing into wafers, as with conventional silicon and gallium arsenide substrates; (ii) producing high-quality crystals, as characterized by low concentrations of impurities and dislocations and narrow x-ray rocking curves; and (iii) growth rates that are high enough for moderate-cost production.
The technique for bulk AlN growth that has probably provided the largest crystals grown to date is sublimation-recondensation, as disclosed originally by Slack and McNelly [J. Cryst. Growth 34, 263 (1976) and 42, 560 (1977)]. However, this technique requires extremely high temperatures (ca. 2250° C.), involves contact with a crucible, which can cause stresses and contamination, and tends to produce crystals with substantial concentrations of dislocations, low-angle grain boundaries, and polycrystalline domains. Several authors have disclosed growth of small AlN crystals in supercritical ammonia, notably Peters [J. Cryst. Growth 104, 411 (1990)], Kolis et al. [Mater. Res. Soc. Symp. Proc. 495, 367 (1998)], Dwilinski et al. [Diamond Relat. Mater. 7, 1348 (1998)], and Demazeau et al. [High Pressure Research 18, 227 (2000)]. However, these latter approaches were limited by the type of apparatus available (autoclaves) to a maximum pressure of 2-5 kbar and a maximum temperature of 400-600° C., produced crystals in the micron size range, and the growth rate was very low. AlN growth in a liquid flux, such as Ca3N2 [U.S. Pat. No. 3,450,499] or Na3AlF6 [R B Campbell et al., Technical Report AFAPL-TR-67-23 (1967)] has also been reported, but in each case the crystals produced were no larger than 1 mm.
Single crystal growth of InN has not been reported by any method, to the best of our knowledge. The inventors have disclosed methods for growing high-quality GaN crystals in supercritical ammonia. However, straightforward application of these methods to the growth of AlN were unsuccessful. Al and In are both more oxophilic than Ga, and the present inventors have discovered that reduction of the oxide contents of the raw material was necessary in order for successful crystal growth in supercritical ammonia.
Techniques for preparing high quality nitride crystal are provided herein.